MCAT Amino Acids: The Complete Cheat Sheet (Memorize in One Week)
Amino acids appear on virtually every MCAT exam — in C/P (titration curves, pI calculations), B/B (protein structure, enzyme function), and even P/S (neurotransmitter synthesis). If you know the 20 amino acids cold, you're guaranteed easy points across multiple sections.
Here's exactly what you need to memorize and the fastest way to do it.
The 4 Groups You Must Know
Nonpolar / Hydrophobic (8 amino acids)
Glycine, Alanine, Valine, Leucine, Isoleucine, Proline, Methionine, Wtryptophan + Fphenylalanine
Memory trick: "GAV LIP MW F" — Gav's lip is MWF (Monday, Wednesday, Friday)
These amino acids are found in the interior of globular proteins (away from water) and in transmembrane domains of membrane proteins.
Polar / Uncharged (5 amino acids)
Serine, Threonine, Cysteine, Asparagine, Qglutamine + Ytyrosine
Key facts:
- Serine & Threonine: Phosphorylation sites (kinase targets). This is the #1 way cells regulate enzyme activity.
- Cysteine: Forms disulfide bonds (S-S). Critical for protein tertiary/quaternary structure.
- Asparagine: N-linked glycosylation site.
- Tyrosine: Can be phosphorylated AND is a precursor for thyroid hormones and catecholamines.
Positively Charged at pH 7.4 (3 amino acids)
Klysine (pKa ~10.5), Rarginine (pKa ~12.5), Histidine (pKa ~6.0)
Key facts:
- Histidine is the most tested amino acid on the MCAT. Its pKa (~6.0) is near physiological pH, so it can act as both a proton donor and acceptor — perfect for enzyme active sites.
- At pH 7.4, histidine is mostly deprotonated (neutral). At pH 6.0, it's 50/50.
- Lysine and arginine are almost always positively charged at physiological pH.
Negatively Charged at pH 7.4 (2 amino acids)
Daspartate (pKa ~3.65), Eglutamate (pKa ~4.25)
Both are negatively charged at pH 7.4. Glutamate is also the most abundant excitatory neurotransmitter in the brain (relevant for P/S section).
The 5 Special Amino Acids
These come up in specific question types:
- Glycine — Smallest amino acid (H as side chain). Only amino acid that is NOT chiral (no stereocenter). Fits in tight spaces in collagen.
- Proline — Cyclic structure. Causes kinks in alpha-helices. Abundant in collagen (every third residue: Gly-X-Pro).
- Cysteine — Disulfide bonds. Reducing agents like beta-mercaptoethanol break disulfide bonds (used in SDS-PAGE).
- Histidine — pKa near 7. Buffer at physiological pH. Found in enzyme active sites (catalytic triad of serine proteases).
- Tryptophan — Largest amino acid. Absorbs UV at 280 nm (used to measure protein concentration). Precursor to serotonin and melatonin.
Isoelectric Point (pI) Calculations
The MCAT loves pI calculations. The rule is simple:
- Neutral amino acid: pI = average of the two backbone pKa values (amino + carboxyl)
- Acidic amino acid (D, E): pI = average of the two lowest pKa values
- Basic amino acid (K, R, H): pI = average of the two highest pKa values
Example: Lysine has pKa values of 2.2 (carboxyl), 9.0 (amino), and 10.5 (side chain). pI = (9.0 + 10.5) / 2 = 9.75
How to Memorize All 20 in One Week
Use spaced repetition with visual cards. DoctorMCAT's Amino Acid flashcard deck includes:
- All 20 structures with side chain classifications
- pKa values and pI calculations
- Special properties and MCAT-specific facts
- Spaced repetition scheduling so you review at optimal intervals
Day 1-2: Learn the groups and one-letter codes. Day 3-4: Learn structures and pKa values. Day 5-6: Practice pI calculations. Day 7: Full self-test. By day 7, you should be able to draw all 20 structures from memory.